1. Field of the Invention
The present invention relates to a semiconductor lithography technique. In particular, the present invention relates to an exposure monitor mask for highly accurately monitoring an effective exposure value to obtain the maximum exposure margin. Further, the present invention relates to an exposure control method, and a method of manufacturing a semiconductor device using the foregoing exposure monitor mask.
2. Description of the Related Art
Short wavelength of exposure wavelength and high NA of projection lens have been required with the scale down of patterns, and simultaneously, the process improvement has been made. However, the requirements of the scale down of device patterns are becoming recently stricter. It is difficult to sufficiently obtain a degree of freedom of exposure and an exposure margin of the depth of focus; as a result, the yield is reduced. In order to effectively use less exposure margin to prevent the reduction of the yield, it is required to high-accurately control exposure and focus.
The exposure control method is usually determined by measuring a line width of pattern. However, the pattern line width varies depending on not only exposure but also focus. The scale down of patterns further advances, and thereby, focus error largely affects the pattern line width. For this reason, it is difficult to determine whether the variation of the pattern line width result from variations of proper exposure value or focus position. Thus, the method of high accurately controlling the exposure is required.
Contrary, Starikov discloses the method of measuring an effective exposure receiving no influence by focus variations (Alexander Starikov, SPIE Vol. 1261 Integrated Circuit Metrology, Inspection, and Process Control IV (1990) p. 315). Starikov proposes a mask pattern for the exposure monitor mark such that the focus error gives no influence to the line width. According to the Starikov proposal, a block having a pattern width that is not resolved in a projection exposure apparatus (aligner) is used. The block is arranged to continuously vary a dimension ratio (duty ratio) of light transmission and shield portions of the pattern. By doing so, a mark having an irradiation gradient distribution, which does not depend on a focus state, is formed on a wafer. In other words, only zero-order diffracted light passing the vicinity of the center of the lens within the NA is focused in a diffracted light image of the mask pattern for the exposure monitor mark. By doing so, effective exposure is monitored without receiving the influence by defocus.
The scale down further advances, and in addition, a process of using an alternating phase shift mask increases. For this reason, when the mask pattern for the exposure monitor mark is formed in the alternating phase shift mask, there are new problems, which do not so far arise in normal binary mask or half-tone phase shift mask.
When the normal binary mask or half-tone phase shift mask is used, diffracted light is generated from mask pattern corresponding to device pattern to be monitored on the mask. In the diffracted light, zero-order and± first-order diffracted light contributes to image formation (imaging) on the wafer. The diffracted light strength has a relation of zero order ≧± first order. The± first-order diffracted light has shading portions at the edge portion of pupil. For this reason, the zero-order diffracted light largely contributes as exposure. The exposure monitor mark is formed by the mask pattern for the exposure monitor mark in which only zero-order diffracted light is projected on the wafer. Thus, the dimension of the exposure monitor mark is measured to calculate variations of the effective exposure. From the calculated variations, the variations described below are high accurately monitored. One is an exposure apparatus dose variation. Another is post exposure bake (PEB) temperature variation and PEB time variation. Another is a resist sensitivity variation. Another is an effective exposure variation by the change of resist or front-end film thickness resulting from standing wave effect.
However, in the alternating phase shift mask, no zero-order diffracted light of the lens of actual device mask pattern is generated due to alternating phase shift effect. The foregoing± first-order diffracted light forms a latent image of the actual device pattern. A conventional exposure monitor mark is formed by only zero-order diffracted light passing the vicinity of the center of the projection lens. The incident angle onto the resist film is different between diffracted light for forming the latent image of the exposure monitor mark and the latent image of the actual device pattern.
As a result, if the effective exposure variation is monitored using the exposure mask mark, it is possible to monitor variations resulting from of the foregoing exposure apparatus dose, PEB temperature and PEB time. However, in the variation resulting from the foregoing resist or front-end film thickness variation, the incident angle onto the resist is largely different in formation between the mask pattern for the exposure monitor mark and actual pattern. For this reason, the influence by standing wave effect is different; therefore, the effective exposure with respect to the actual pattern is not monitored highly accurately.